The use of resin ink compositions to form thick film resistors on printed circuit boards is well known in the art. Such compositions are described, for example, in U.S. Pat. Nos. 3,648,364 and 2,795,680.
Resin ink compositions for thick film resistors are conventionally comprised of a resin, usually an epoxy resin or a modified epoxy resin, conductive particles, usually carbon and/or graphite, and a suitable organic solvent. U.S. Pat. No. 2,795,680 discloses such compositions which may contain up to about 25% by weight of finely divided nonconductive materials, including alumina.
Heretofore, it has been necessary to utilize an intermediate pad of a conductive material, e.g. silver, between copper foil conductors and the thick film resistors formed from inks such as described above. The use of intermediate pads is required because resistors formed from such conventional ink compositions are unstable when terminated directly to copper foil and will often begin to deteriorate in use within a matter of hours. This instability, in all probability, is due to the large difference in the thermal coefficient of expansion between the resistors and both the copper foil and the x-y axis of the printed circuit board. Resistors formed from conventional inks also frequently adhere poorly when bonded directly to copper foil conductors.
The use of an intermediate pad of, e.g. silver, usually solves the problems of instability and poor adhesion experienced with resistors formed from conventional thick film inks. However, in addition to the added cost of the silver, the use of such intermediate pads requires the time and cost of formulating appropriate epoxy compositions which must be applied and cured on the circuit board before the resistor film is applied.
It would be desirable to have thick film resistor ink compositions which can be terminated directly to copper foil conductors without the use of silver intermediate pads. In addition, it would be desirable to have thick film resistors which have superior power dissipation capacity than previously known formulations, whether terminated directly to copper foil or connected thereto via silver intermediate pads.